Researchers start with the roots to shrink trees.

An NC-140 peach dwarfing rootstock trial, planted in the spring of 2017, compares industry standards Guardian and Lovell to three interspecific hybrids (Rootpac 40 and 20 and MP-29) and three all-peach rootstocks (Controller 6, 7 and 8). All the rootstocks are planted with Cresthaven at sites in Alabama, Colorado, Georgia, Michigan, North Carolina, New York, Pennsylvania, South Carolina and Ontario, Canada. Note that the MP-29 sample was pruned by the nursery. (Image courtesy of trial coordinator Ioannis Minas, Colorado State University)

Peach growers want to reap the same rewards in orchard efficiency and mechanization potential as apple growers have gained from dwarfing rootstocks, planted in high-density systems.

They aren’t quite there yet.

“Peaches are in a plateau,” said Ioannis Minas, a pomology professor at Colorado State University and coordinator of the latest NC-140 rootstock trial focused on semi-dwarfing peach rootstocks. “Peach is a vigorous tree and we cannot move into intensive plantings with the existing standard rootstocks.”

Complicating the efforts, peach growers require high tolerance to replant conditions, Armillaria root rot and peach tree short life disease. Rootstocks also must be adapted to climate and soil conditions. Previous research trials have focused more on those traits. Researchers have found that rootstocks that perform well in the West are often poor performers in the East, and vice versa, Minas said.

“For peaches, we look for the longevity and health of the trees first and then size control for these intensive systems,” said Greg Reighard, a recently retired Clemson horticulture professor and longtime coordinator of the NC-140 trials for peach. “We’re making good progress.”

It’s hard to find both traits in the same genotype because often, the vigor that helps a tree tolerate replant or other adverse conditions also makes it hard to manage the tree productively, Minas said.

Broadly, rootstock options include interspecific hybrids, such as peach-plum or peach-almond hybrids; peach-peach hybrids such as the Controller series; and peach seedling rootstocks, such as industry standards Lovell and Guardian, which offers protection against peach tree short life disease so that “the trees are living long enough to get Armillaria,” Reighard said.

The interspecific hybrids tend to perform well in western type soils with higher calcium levels that peach trees are very sensitive to, Minas said. Plum hybrids also perform better in wet ground, Reighard said.

Right now, Krymsk 86, a plum-peach hybrid bred in Russia, is the most popular new rootstock for peach growers on the Colorado Plateau, Minas said. It’s a full-size tree that performs well in replant situations, which is key in many peach growing regions where there are few alternatives for new ground.

The latest trial, planted in the spring of 2017, includes three interspecific hybrids that promise size control. Two, Rootpac 20 and Rootpac 40, were developed in Spain, and while they are performing well for U.S. almond growers, this will be their first trial on peach. All the rootstocks in the NC-140 trial are planted with Cresthaven in a perpendicular V system at sites in Alabama, Colorado, Georgia, Michigan, North Carolina, New York, Pennsylvania, South Carolina and Ontario, Canada.

The other hybrid is MP-29, a semi-dwarfing plum-peach hybrid developed by the U.S. Department of Agriculture in Georgia. It also features resistance to Armillaria, which is critical in the Southeast, and resistance to peach tree short life comparable to industry standard Guardian, but at 70 percent the size.

It’s very promising in terms of orchard performance, but hard to produce, Reighard said. “We’re trying to find a rootstock that’s easier to propagate and that might have more resistance,” he said. Genetic research into disease resistance is a major focus right now at Clemson and elsewhere, he added.

The all-peach rootstocks in the trial come from the University of California Davis breeding program. Controller 6, 7 and 8 offer trees with 60, 70 and 80 percent the size of Nemaguard, respectively, Minas said. They are also resistant to root knot nematode. The previous NC-140 trial featured Controller 5, which produced smaller fruit, but California tests on 6, 7 and 8 seem more promising in terms of small trees and big fruit.

In Pennsylvania, horticulturist Jim Schupp is excited about the Controller series, in particular Controller 8, he told growers at the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan, in December.

“We are pretty bullish on this rootstock and I’d like to get more trees on this rootstock,” the Penn State University professor said. “It’s one of the ways we could get back to a pedestrian orchard.”

The two controls included in the trial are industry standards Lovell and Guardian. Over the next several years, the NC-140 trial will look at survival, sucker production, tree size, fruit production, fruit quality and cold hardiness. Minas urged growers to attend field days in their local area to see the trees for themselves. All the rootstocks are already commercially available, but he expects most growers to wait for the trial to vet them before planting on a large scale.

“We want to find smaller trees that can be adopted for more high-density systems. We believe we need to improve mechanization for peach production, and with existing rootstocks, we can’t move in that direction,” Minas said. •